Why does your hot water run out quicker than it used to with a gas water heater?

It shouldn't. Gas continually heats the water very quickly, so you don't really run out of hot water at all, where with regular heating it takes a long time to heat new water. You should get the person who installed the system to doublecheck, and possibly they can explain why it is happening.

It could be the dip tube of broken or you could have alot of hardness build-up in the bottom of the tank.